The march toward government shutdown got its start in mid-July with a meeting in the office of Senator Mike Lee of Utah. Lee and his colleagues Ted Cruz of Texas and Marco Rubio of Florida huddled with representatives of Tea Party groups like Heritage Action, ForAmerica, and the Tea Party Patriots. Their mission: to turn defunding Obamacare from a fringe idea with little congressional support into a popular movement.

It worked. The groundswell the conservative groups helped create over the summer make conservatives in Congress aware of the fight, in turn forcing the hand of Republican leaders. Obamacare has not, of course, been defunded, but it was House Republicans' insistence on crippling the health-care bill as a condition of funding the government that brought about this week's shutdown.

The real reason the House GOP hasn’t backed down and passed a government-funding bill isn’t because of 30-some intractable Tea Party members in their ranks or because Cruz refused to play along. It’s because their loudest, most engaged constituents demanded it, amplified by the savvy, coordinated tactics of the right-wing pressure groups that have proved adept at leveraging grassroots pressure into Washington results. As Representative Greg Walden recently told a group of Republican donors, according to the Daily Beast: “We have to do this because of the Tea Party .... The Tea Party gets involved at the local level."

President Obama’s campaign ground game helped him win reelection; afterward, his supporters vowed to bring some of those tactics to bear on pressuring Congress to enact his agenda. But it’s the Tea Party that has mastered this game, through a combination of savvy tactics and ruthless persistence.

"Republicans had never made a serious challenge to Obamacare. We concluded this was when it had to happen."

At the time of the meeting in Lee's office, Congress was preparing for its summer vacation, and conservatives faced a dispiriting reality: On October 1, the Obamacare exchanges would open for business. When that happened, “The toothpaste would be out of the tube, and it would be be very difficult to put an end to this,” said Brent Bozell, president of ForAmerica, a little-known but influential conservative group that relies mainly on social media. “It was a last-gasp effort.”

They knew where the pressure points were. Republicans owe their House majority to the Tea Party wave of 2010, which was spurred in turn largely by reaction to the health-care bill. The Tea Party has proved less than adept at winning general elections (see: Obama, President). But it has shown abundantly that it knows how to put the heat on in primaries (see: O'Donnell, Christine). With redistricting largely protecting incumbents in general elections, a primary threat is the biggest fear for most members of Congress.

So the Tea Party groups went on the attack. ForAmerica was the most aggressive, making videos calling out senators as “chicken” — including Minority Leader Mitch McConnell and South Carolina Senator Lindsey Graham, both of whom face Tea Party challengers in primaries next year. Texas Senator John Cornyn, who is up for reelection but doesn’t have a challenger, also got one. The activists knew they didn’t have to make an ad for every Republican senator; once a few got the treatment, the rest got the message. Nobody wanted to be next on the list.

“Republicans who had been committing themselves to putting an end to this wretched law since it passed really weren’t doing anything,” Bozell told me. “You had people telling their constituents, ‘I voted against it 39 times’ when those were just meaningless, procedural votes. They'd never made a serious challenge to Obamacare. We concluded this was when it had to happen.”

ForAmerica’s 3.5 million Facebook fans were instructed to call the offices of Republican leaders. They shut down the phone lines of Speaker John Boehner, McConnell, Cornyn, Majority Leader Eric Cantor, Majority Whip Kevin McCarthy, and others. When one office line stopped working, they’d redirect callers to a different one. The group says it generated 60,000 calls.

Other groups played complementary roles. Heritage Action drew hundreds to town halls in nine cities. ForAmerica and the Tea Party Patriots teamed up for a tour of a dozen states, the locations strategically chosen for maximum impact — like a rally in Lexington, Kentucky, to target McConnell — and culminating on the lawn of the Capitol. Meanwhile, Tea Party Patriots members held hundreds of rallies and protests in their home districts. Many were mock town halls featuring an empty chair to represent a member of congress who wasn’t meeting with constituents.

Rallies, protests, town halls, email petitions, call-in campaigns: None of these tactics are rocket science. But deployed strategically, with the coordination enabled by technology, they work. And they're cheap: ForAmerica and the Tea Party Patriots each estimated their campaigns cost about $200,000.

To be sure, the groups' achievement falls far short of victory.While the campaigns have succeeded at bending the Washington debate, they haven't achieved a policy goal — to the contrary, the exchanges opened on schedule this week — nor have they won converts across the aisle. The Tea Party's clout shows both the possibilities and the limitations of grassroots pressure on Congress — a tactic that advocates of a different persuasionhave tried and failed to mobilize behind goals like gun control and immigration reform.

Where the right is concerned, however, the Tea Party's power shows no sign of waning. If anything, this battle appears to have emboldened the far right, a fact Boehner and his lieutenants have no doubt absorbed as they look ahead. Even if the speaker ignores the conservative faction of his caucus on the present issue, he will not have tamed this force. Bozell has no qualms about the heartburn he's caused for the GOP: “I could give a damn," he said. "I’m fed up with the Washington elites of both parties.”

The Tea Party Patriots runs on a bottom-up model, with hundreds of local coordinators joining a Sunday-evening web conference every week to decide strategy. Jenny Beth Martin, the group's national coordinator, doesn’t decide what positions the group will take; she puts the question to the local coordinators, who vote online. In this case, she said, the decision was nearly unanimous: “The position was, Don’t spend money on Obamacare — and don’t blink,” Martin said.

The group, she noted, has been working to refine its tactics since the disappointment of the 2012 election (during which, they maintain, Obamacare was never properly debated). Two books detailing Obama campaign tactics, The Audacity to Win, by 2008 campaign manager David Plouffe, and journalistSasha Issenberg’s The Victory Lab, have been assigned as reading.

The group commissioned a pollster to collect data and test messaging. That’s why, Martin says, you’ve heard such a consistent refrain from Tea Party activists when it comes to Obamacare: The message is about lost jobs, lost coverage, lost hours, with much less of the old, alienating rhetoric about creeping socialism or death panels.

Having hammered home the message that politicians would be punished for crossing the grassroots, the activists have a positive reinforcement to offer as well. Now that Republican leadership has followed their lead and (mostly) held the line, they’re being lavished with praise. “They have listened to the will of the people,” Martin says. “They are showing that in our government there is a check and a balance.” To those who argue that Obamacare is the law of the land, she argues, “We’ve pulled back from [demanding] full repeal. We understand that elections do have consequences. But the political reality today is that the American public voted to divide power in this country.”

With the government shut down, the message to House leaders from the Tea Party is: We’ve got your back. To those who argue that the Obamacare battle can’t be won, they have a ready answer: Look how far we’ve already come.

“It hasn’t happened yet, but the debate is now taking place on Obamacare, and that would not have happened but for us,” Bozell said. “The leaders of both houses were prepared to surrender unequivocally, but now they’re fighting on it, at least in the House.”